postalsys/mailauth

Command line utility and a Node.js library for email authentication.

• SPF verification
• DKIM signing
• DKIM verification
• DMARC verification
• ARC verification
• ARC sealing
  • Sealing on authentication
  • Sealing after modifications
• BIMI resolving and VMC validation
• MTA-STS helpers

Pure JavaScript implementation, no external applications or compilation needed. It runs on any server/device that has Node 16+ installed.

Command line usage

See command line documentation for the mailauth command.

Library Usage

Authentication

Validate DKIM signatures, SPF, DMARC, ARC, and BIMI for an email.

await authenticate(message [,options]) ->
    { dkim, spf, arc, dmarc, bimi, receivedChain, headers }

Where

• message is either a String, a Buffer, or a Readable stream that represents an email message
• options (object) is an optional options object
  • sender (string) is the email address from MAIL FROM command. If not set, then it is parsed from the Return-Path header
  • ip (string) is the IP of the remote client that sent this message
  • helo (string) is the hostname value from HELO/EHLO command
  • trustReceived (boolean) if true, then parses values for ip and helo from the latest Received header if you have not set these values yourself. Defaults to false.
  • mta (string) is the hostname of the server performing the authentication (defaults to os.hostname())
  • minBitLength (number) is the minimum allowed bits of RSA public keys (defaults to 1024). If a DKIM or ARC key has fewer bits, then validation is considered as failed
  • disableArc (boolean) if true then skip ARC checks
  • disableDmarc (boolean) if true then skip DMARC checks. It also disables checks that are dependent on DMARC (e.g., BIMI)
  • disableBimi (boolean) if true, then skip BIMI checks
  • seal (object) if set and message does not have a broken ARC chain, then seals the message using these values
    • signingDomain (string) ARC key domain name
    • selector (string) ARC key selector
    • privateKey (string or buffer) Private key for signing. Either an RSA or an Ed25519 key
  • resolver (async function) is an optional async function for DNS requests. Defaults to dns.promises.resolve
  • maxResolveCount (number defaults to 10) is the DNS lookup limit for SPF. RFC7208 requires this limit to be 10.
  • maxVoidCount (number defaults to 2) is the DNS lookup limit for SPF that produce an empty result. RFC7208 requires this limit to be 2.

Example

const { authenticate } = require('mailauth');
const { dkim, spf, arc, dmarc, bimi, receivedChain, headers } = await authenticate(
    message, // either a String, a Buffer or a Readable Stream
    {
        // SMTP transmission options if available
        ip: '217.146.67.33', // SMTP client IP
        helo: 'uvn-67-33.tll01.zonevs.eu', // EHLO/HELO hostname
        sender: '[email protected]', // MAIL FROM address

        // Uncomment if you do not want to provide ip/helo/sender manually but parse from the message
        //trustReceived: true,

        // Server processing this message, defaults to os.hostname(). Inserted into Authentication headers
        mta: 'mx.ethereal.email',

        //  Optional  DNS resolver function (defaults to `dns.promises.resolve`)
        resolver: async (name, rr) => await dns.promises.resolve(name, rr)
    }
);
// output authenticated message
process.stdout.write(headers); // includes terminating line break
process.stdout.write(message);

Example output:

Received-SPF: pass (mx.ethereal.email: domain of [email protected] designates 217.146.67.33 as permitted sender) client-ip=217.146.67.33;
Authentication-Results: mx.ethereal.email;
 dkim=pass [email protected] header.s=default header.a=rsa-sha256 header.b=TXuCNlsq;
 spf=pass (mx.ethereal.email: domain of [email protected] designates 217.146.67.33 as permitted sender) [email protected]
 smtp.helo=uvn-67-33.tll01.zonevs.eu;
 arc=pass (i=2 spf=neutral dkim=pass dkdomain=ekiri.ee);
 dmarc=none header.from=ekiri.ee
From: ...

You can see the full output (structured data for DKIM, SPF, DMARC, and ARC) from this example.

receivedChain

receivedChain property is an array of parsed representations of the Received: headers.

DKIM

Signing

const { dkimSign } = require('mailauth/lib/dkim/sign');
const signResult = await dkimSign(
    message, // either a String, a Buffer or a Readable Stream
    {
        // Optional, default canonicalization, default is "relaxed/relaxed"
        canonicalization: 'relaxed/relaxed', // c=

        // Optional, default signing and hashing algorithm
        // Mostly useful when you want to use rsa-sha1, otherwise no need to set
        algorithm: 'rsa-sha256',

        // Optional, default is current time
        signTime: new Date(), // t=

        // Keys for one or more signatures
        // Different signatures can use different algorithms (mostly useful when
        // you want to sign a message both with RSA and Ed25519)
        signatureData: [
            {
                signingDomain: 'tahvel.info', // d=
                selector: 'test.rsa', // s=
                // supported key types: RSA, Ed25519
                privateKey: fs.readFileSync('./test/fixtures/private-rsa.pem'),

                // Optional algorithm, default is derived from the key.
                // Overrides whatever was set in parent object
                algorithm: 'rsa-sha256',

                // Optional signature specifc canonicalization, overrides whatever was set in parent object
                canonicalization: 'relaxed/relaxed' // c=

                // Maximum number of canonicalized body bytes to sign (eg. the "l=" tag).
                // Do not use though. This is available only for compatibility testing.
                // maxBodyLength: 12345
            }
        ]
    }
); // -> {signatures: String, errors: Array} signature headers using \r\n as the line separator
// show signing errors (if any)
if (signResult.errors.length) {
    console.log(signResult.errors);
}
// output signed message
process.stdout.write(signResult.signatures); // includes terminating line break
process.stdout.write(message);

Example output:

DKIM-Signature: a=rsa-sha256; v=1; c=relaxed/relaxed; d=tahvel.info;
 s=test.rsa; b=...
From: ...

Verifying

const { dkimVerify } = require('mailauth/lib/dkim/verify');
// `message` is either a String, a Buffer or a Readable Stream
const result = await dkimVerify(message);
for (let { info } of result.results) {
    console.log(info);
}

Example output:

dkim=neutral (invalid public key) [email protected] header.s=test.invalid header.b="b85yao+1"
dkim=pass [email protected] header.s=test.rsa header.b="BrEgDN4A"
dkim=policy policy.dkim-rules=weak-key [email protected] header.s=test.small header.b="d0jjgPun"

SPF

Verifying

const { spf } = require('mailauth/lib/spf');

let result = await spf({
    sender: '[email protected]',
    ip: '217.146.76.20',
    helo: 'foo',
    mta: 'mx.myhost.com'
});
console.log(result.header);

Example output:

Received-SPF: pass (mx.myhost.com: domain of [email protected]
 designates 217.146.76.20 as permitted sender) client-ip=217.146.76.20;
 envelope-from="[email protected]";

ARC

Validation

ARC seals are automatically validated during the authentication step.

const { authenticate } = require('mailauth');
const { arc } = await authenticate(
    message, // either a String, a Buffer or a Readable Stream
    {
        trustReceived: true
    }
);
console.log(arc);

The output is something like this:

{
  "status": {
    "result": "pass",
    "comment": "i=2 spf=neutral dkim=pass dkdomain=zonevs.eu dkim=pass dkdomain=srs3.zonevs.eu dmarc=fail fromdomain=zone.ee"
  },
  "i": 2,
  ...
}

Sealing

During authentication

You can seal messages with ARC automatically in the authentication step by providing the sealing key. In this case, you can not modify the message any more as this would break the seal.

const { authenticate } = require('mailauth');
const { headers } = await authenticate(
    message, // either a String, a Buffer or a Readable Stream
    {
        trustReceived: true,

        // ARC seal settings. If this is set then resulting headers include
        // a complete ARC header set (unless the message has a failing ARC chain)
        seal: {
            signingDomain: 'tahvel.info',
            selector: 'test.rsa',
            privateKey: fs.readFileSync('./test/fixtures/private-rsa.pem')
        }
    }
);
// output authenticated and sealed message
process.stdout.write(headers); // includes terminating line break
process.stdout.write(message);

After modifications

If you want to modify the message before sealing, you have to authenticate the message first and then use authentication results as input for the sealing step.

const { authenticate, sealMessage } = require('mailauth');

// 1. authenticate the message
const { arc, headers } = await authenticate(
    message, // either a String, a Buffer or a Readable Stream
    {
        ip: '217.146.67.33', // SMTP client IP
        helo: 'uvn-67-33.tll01.zonevs.eu', // EHLO/HELO hostname
        mta: 'mx.ethereal.email', // server processing this message, defaults to os.hostname()
        sender: '[email protected]' // MAIL FROM address
    }
);

// 2. perform some modifications with the message ...

// 3. seal the modified message using the initial authentication results
const sealHeaders = await sealMessage(message, {
    signingDomain: 'tahvel.info',
    selector: 'test.rsa',
    privateKey: fs.readFileSync('./test/fixtures/private-rsa.pem'),

    // values from the authentication step
    authResults: arc.authResults,
    cv: arc.status.result
});

// output authenticated message
process.stdout.write(sealHeaders); // ARC set
process.stdout.write(headers); // authentication results
process.stdout.write(message);

DMARC

DMARC is verified as part of the authentication process and even as the dmarc handler is exported, it requires input from previous steps.

Helpers

getDmarcRecord(domain [,resolver])

Returns parsed DMARC DNS record for a domain or a subdomain or false is no record exists.

const getDmarcRecord = require('mailauth/lib/dmarc/get-dmarc-record');
const dmarcRecord = await getDmarcRecord("ethereal.email");
console.log(dmarcRecord);

Output

{
  v: 'DMARC1',
  p: 'none',
  pct: 100,
  rua: 'mailto:[email protected]',
  sp: 'none',
  aspf: 'r',
  rr: 'v=DMARC1; p=none; pct=100; rua=mailto:[email protected]; sp=none; aspf=r;',
  isOrgRecord: false
}

isOrgRecord is true for sudomains, where organizational domain's DMARC policy applies, so use the sp, not p policy.

Optionally set resolver argument with custom resolver (uses dns.resolve by default).

BIMI

Brand Indicators for Message Identification (BIMI) support is based on draft-blank-ietf-bimi-01.

BIMI information is resolved in the authentication step, and the results can be found from the bimi property. The message must pass DMARC validation to be processed for BIMI. DMARC policy can not be "none" for BIMI to pass.

const { bimi } = await authenticate(
    message, // either a String, a Buffer or a Readable Stream
    {
        ip: '217.146.67.33', // SMTP client IP
        helo: 'uvn-67-33.tll01.zonevs.eu', // EHLO/HELO hostname
        mta: 'mx.ethereal.email', // server processing this message, defaults to os.hostname()
        sender: '[email protected]', // MAIL FROM address

        bimiWithAlignedDkim: false // If true then ignores SPF in DMARC and requires a valid DKIM signature
    }
);
if (bimi?.location) {
    console.log(`BIMI location: ${bimi.location}`);
}

BIMI-Location header is ignored by mailauth, it is not checked for, and it is not modified in any way if it is present. BIMI-Selector is used for selector selection (if available).

Verified Mark Certificate

Authority Evidence Document location is available from the bimi.authority property (if set).

VMC (Verified Mark Certificates) for Authority Evidence Documents is a X509 certificate with an id-pe-logotype extension (oid=1.3.6.1.5.5.7.1.12) that includes a compressed SVG formatted logo file (read more here).

Some example authority evidence documents:

• from default._bimi.cnn.com
• from default._bimi.entrustdatacard.com

MTA-STS

mailauth allows you to fetch MTA-STS information for a domain name.

const { getPolicy, validateMx } = require('mailauth/lib/mta-sts');

let knownPolicy = getCachedPolicy('gmail.com'); // optional
let mx = 'alt4.gmail-smtp-in.l.google.com';

const { policy, status } = await getPolicy('gmail.com', knownPolicy);
const policyMatch = validateMx(mx, policy);

if (policy.id !== knownPolicy?.id) {
    // policy has been updated, update cache
}

if (policy.mode === 'enforce') {
    // must use TLS
}

if (policy.mx && !policyMatch.valid) {
    // can't connect, unlisted MX
}

Resolve policy

Resolve MTA-STS policy for a domain

async getPolicy(domain [,knownPolicy]) -> {policy, status}

Where

• domain is the domain to check for (e.g. "gmail.com")
• knownPolicy (optional) is the policy object from the last check for this domain. This is used to check if the policy is still valid or it was updated.

The function returns an object with the following properties:

• policy (object)
  • id (string or false) ID of the policy
  • mode (string) one of "none", "testing" or "enforce"
  • mx (array, if available) an Array of whitelisted MX hostnames
  • expires (string, if available) ISO date string for cacheing
• status (string) one of the following values:
  • "not_found" no policy was found for this domain. You can decide yourself how long you want to cache this response
  • "cached" no changes detected, current policy is still valid and can be used
  • "found" new or updated policy was found. Cache this in your system until policy.expires
  • "renew" existing policy is still valid, renew cached version until policy.expires
  • "errored" policy discovery failed for some temporary error (e.g., failing DNS queries). See policy.error for details

Validate MX hostname

Check if a resolved MX hostname is valid by MTA-STS policy or not.

validateMx(mx, policy) -> Object

Where

• mx is the resolved MX hostname (eg. "gmail-smtp-in.l.google.com")
• policy is the policy object returned by getPolicy()

The function returns an object. If {valid} is true, then MX hostname is allowed to be used.

Testing

mailauth uses the following test suites:

SPF test suite

OpenSPF test suite (archive.org mirror) with the following differences:

• Less strict whitespace checks (mailauth accepts multiple spaces between tags etc.)
• Some macro tests are skipped (macro expansion is supported in most parts)
• Some tests where the invalid component is listed after a matching part (mailauth processes from left to right and returns on the first match found)
• Other than that, all tests pass

ARC test suite from ValiMail

ValiMail arc_test_suite

• mailauth is less strict on header tags and casing. For example, uppercase S= for a selector passes in mailauth but fails in ValiMail.
• Signing test suite is used for input only. All listed messages are signed using provided keys, but signatures are not matched against the reference. Instead, mailauth validates the signatures itself and looks for the same cv= output that the ARC-Seal header in the test suite has
• Other than that, all tests pass

Setup

First, install the module from npm:

$ npm install mailauth

next import any method you want to use from mailauth package into your script:

const { authenticate } = require('mailauth');

License

© 2020-2022 Postal Systems OÜ

Licensed under MIT license